Typically, the invention of this type includes, as described in, e.g., Patent Literature 1, a clutch device including an output shaft (2), a clutch disc (38) and an input disc (28) supported on an outer peripheral portion of the output shaft, and a coil spring (44) configured to bias the clutch disc and the input disc such that the clutch disc and the input disc are pressed against each other. Note that a numerical value in parentheses indicates a reference numeral in Patent Literature 1.
In this clutch device, one end surface of the clutch disc is provided with a trapezoidal engagement recessed portion (42), and an opposing surface of the input disc is provided with a substantially ridge-shaped engagement raised portion (37). When an excessive rotation load acts on the output shaft, an inclined surface of the engagement recessed portion and an inclined surface of the engagement raised portion slide against each other by the rotation load, and in this manner, these portions in a fitted state are disengaged. Such a clutch device is sometimes referred to as a “torque limiter” or a “safety clutch,” for example.
According to such a typical technique, in a case where the rotation load is rapidly applied to the output shaft, a case where the rotation load is extremely larger than expected, a case where the rotation load is repeatedly received multiple times, and the like, there is a probability that the engagement raised portion is deformed or damaged. For this reason, durability of the engagement raised portion might be increased by a material change. However, this leads to a cost increase.